The pelvis, so called from its resemblance to a basin, is a massive bony ring interposed between the movable segments of the vertebral column which it supports, and the lower limbs upon which it rests ; it is composed of the two hip-bones laterally and in front, and the sacrum and coccyx behind. It is divided into the false (greater) and the true (lesser) pelvis; by an oblique plane passing through the prominence of the sacrum behind, and the arcuate line on each side and in front. The circumference of this plane is termed the pelvic inlet (pelvic brim).
The false pelvis (greater pelvis), the expanded portion of the cavity above and in front of the pelvic inlet, is formed on each side by the ilium and posteriorly by the base of the sacrum.
The true pelvis (lesser pelvis) is that part of the pelvic cavity which lies below and behind the pelvic inlet. Its bony walls are more complete than those of the false pelvis. It possesses an inlet bounded by the superior circumference, an outlet bounded by the inferior circumference, and a cavity.
The superior circumference forms the brim of the pelvis, the included space being called the inlet (fig. 448). The inlet is somewhat heart-shaped, obtusely pointed in front, and encroached upon behind by the forward projection of the promontory of the sacrum. It has three principal diameters : anteroposterior, transverse, and oblique. The anteroposterior or conjugate diameter extends from the lumbosacral angle to the symphysis pubis ; its average measurement is about 110 mm. in the female. The transverse diameter extends from the middle of the brim: on one side to the same point of the opposite side; its average measurement is about 135 mm in the female. The oblique diameter extends from the iliopubic (iliopectincal) eminence to the opposite sacro-iliac articulation ; its average measurement is about 125 mm in the female.
The anteroposterior diameter of the outlet of the pelvis extends from the apex of the coccyx to the lower part of the pubic symphysis ; its measurement is from 90 to 115 mm. in the female. It varies with the length of the coccyx, and is capable of increase or diminution, on account of the mobility of that bone. The transverse diameter, measured between the posterior parts of the ischial tuberosities, is about 115 mm. in the female.*
Axes (fig. 450).-The axis of the inlet, i.e. a line at right angles to the plane of the inlet through its center, is directed downwards and backwards, and if the line be prolonged it passes through the umbilicus above and the middle of the coccyx below. The axis of the outlet is directed downwards and slightly backwards ; if prolonged upwards it touches the base of the sacrum. The axis of the cavity-i.e. an axis at right angles to a series of planes between and including those of the inlet and outlet-is curved like the cavity itself : this curve is parallel to that of the sacrum and coccyx.
In the sitting posture the body rests on the medial and lower parts of the ischial tuberosities, and a coronal plane drawn through the anterior superior iliac spines passes through the acetabula. The lumbosacral angle is considerably reduced and the projection of the sacral promontory is correspondingly diminished.
Differences between the male and female pelvis (figs. 451, 452, 453,454)-Sexual characters are more pronounced in the bones of the pelvis than in any other bones in the body. The female pelvis is specially adapted to facilitate the passage of the fetal head during parturition and it must therefore provide more accommodation than is necessary in the male pelvis, while its depth must be diminished. The essential differences have been well summed up by Arthur Thomson, who described the male pelvis as a long section of a short cone and the female pelvis as a short section of a long one. There are, however, very many differences in detail, most of which can be referred to these fundamental distinctions.
As a whole, the bones of the female pelvis are more delicate and their muscular impressions are not so well marked. The ilia are more vertical and, although the distance between the iliac crests is less in the female, the anterior superior iliac spines are further apart. As a result, the iliac fossa are shallower and the curves of the crest, as seen from above, are not so pronounced. The prominence of the hips in the female may be attributed, in part, to the sexual characters of the ilium.
The inlet of the true pelvis is larger in the female and is more nearly circular in outline, whereas in the male it is typically heart-shaped.
The cavity of the female pelvis is wider and shallower, and, in the production of this general difference, the following factors are to be noted. (1) The sacrum is shorter and wider in the female and its upper part is straight. (2) The depth of the pubic symphysis is less and the distance between the two pubic tubercles is greater in the female. (3) The sciatic notches are wider and shallower, and the spines of the ischia do not project inwards to the same extent as they do in the male.
The male pelvis possesses one positive sexual character : the margins of the pubic arch are more everted, owing to the larger size of the crura of the penis.
The following additional differences should also be mentioned. (1) The acetabula are smaller in the female: they are wider apart and look more definitely forwards. As a result the transverse diameter of the acetabulum is distinctly less than the distance from its anterior margin to the pubic symphysis. In the male, on the other hand, the two measurements are practically equal. (2) The obturator foramen is smaller in the female and is triangular in shape. In the male it is more oval in outline, but this difference is not of great value as asexual character. (3) The pre-auricular sulcus (p. 372) is more constantly present in the female ilium, and its presence is presumably associated with the existence of a more movable sacro-iliac joint. (4) The auricular surface of the sacrum is limited to the first and second sacral vertebra in the female, but in the male it usually extends to the middle of the third vertebra.
The size of the pelvis varies not only in the two sexes, but also in different members of the same sex, and does not appear to be greatly influenced by the height of the individual. Women of short stature, as a rule, have broad pelves. Occasionally, the pelvis is contracted in all its dimensions, and its diameters may be as much as 12.5 mm. less than the average, and this even in well-formed women of average height. The principal divergences, however, are found at the inlet, and affect the relation of the anteroposterior to the transverse diameter. Thus the inlet may be elliptical either in a transverse or an anteroposterior direction, the transverse diameter in the former, and the anteroposterior in the latter, greatly exceeding the other diameters; in some instances it is almost circular.
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